One of optical fiber amplifiers used for optical fiber communication is an optical fiber Raman amplifier (hereinafter referred to as Raman amplifier).
The Raman amplifier utilizes stimulated Raman effect which is a nonlinear phenomenon in an optical fiber connecting medium. The stimulated Raman effect is a phenomenon that when a substance is irradiated with light having a certain wavelength, light having the same wavelength is scattered, and scattered light having a changed wavelength is generated. For example, when excitation light is incident on the optical fiber connecting medium, scattered light having a wavelength shifted to a long wavelength side by approximately 100 nm compared with the wavelength of the excitation light is obtained. The Raman amplifier utilizes this phenomenon, and amplifies a signal light in the optical fiber connecting medium by guiding excitation light in the vicinity of 1,450 nm, which is a wavelength shifted by approximately 100 nm to a shorter wavelength side from 1,550 nm used as a wavelength band of the signal light.
The optical fiber connecting medium itself is used as an optical amplifying medium, and hence the Raman amplifier is distributed over a long distance to perform amplification, and the power of the signal light is maintained so as not to be reduced. As a result, a low-noise transmission distance can be lengthened. In addition, there is no limitation on an amplifying wavelength region, and hence amplification can be performed at any arbitrary wavelength by setting the wavelength of the excitation light.
FIG. 1 is a structural block diagram illustrating a conventional Raman amplifier. A level of main signal light, to which a loss is caused in an optical fiber connecting medium PL, is recovered owing to stimulated Raman effect of excitation light combined by a wave length division (WDM) coupler P2.
The excitation light is output from a semiconductor excitation laser (excitation laser diode (LD) P4 of FIG. 1, hereinafter referred to as excitation laser or excitation LD) which is an excitation light source. The excitation light is guided in a direction opposite to the transmission direction of the main signal light by the WDM coupler P2. A Raman amplifier P1 outputs the main signal amplified by the excitation light to a subsequent EDFA (not shown).
A gain (Raman gain, ON/OFF gain) of the Raman amplifier is expressed by the following expression on the assumption of a model in which the excitation light attenuates because of fiber loss, and the main signal light is influenced by the loss and the Raman gain.
[Mathematical Expression 1]
                              Signal          ⁢                                          ⁢          light          ⁢                                          ⁢                                    ⅆ                              P                s                                                    ⅆ              z                                      =                                                            g                R                                            A                eff                                      ⁢                          P              p                        ⁢                          P              s                                -                                    α              s                        ⁢                          P              s                                                          (                  Expression          ⁢                                          ⁢          1                )            
[Mathematical Expression 2]
                              Excitation          ⁢                                          ⁢          light          ⁢                                          ⁢                                    ⅆ                              P                p                                                    ⅆ              z                                      =                              -                          α              p                                ⁢                      P            p                                              (                  Expression          ⁢                                          ⁢          2                )            Reference symbol Ps denotes signal light power, which is expressed in dBm. Reference symbol Pp denotes excitation light power, which is expressed in dBm. The excitation light power is light output power of a semiconductor laser for Raman excitation, which is variable by control. Note that the variation in loss is observed in a connected portion to the optical fiber connecting medium for the excitation light, or an optical connector or a splice on the transmission line. Reference symbol αs denotes a fiber loss of the signal light, which is expressed in dB/km. Reference symbol αp denotes a fiber loss of the excitation light, which is expressed in dB/km. Each of αs and αp varies in a range of 0.2 to 0.35 dB/km depending on the optical fiber. Reference symbol gR denotes a Raman gain coefficient. The Raman gain coefficient is a physical property value and changed in accordance with the type of the optical fiber. That is, when the type of the optical fiber is determined, the Raman gain coefficient becomes substantially a constant value. Reference symbol Aeff denotes an effective cross sectional area of the optical fiber, which is expressed in μm2. The effective dross sectional area of the optical fiber is determined based on the type of the optical fiber.
Among them, the excitation light power Pp and the fiber losses αs and αp greatly influence the gain of the Raman amplifier during actual operation. Expression 1 described above indicates that the gain of the Raman amplifier increases with the excitation light power Pp.
The excitation laser P4 of the Raman amplifier P1 has an upper limit value and a lower limit value for the excitation light which can be output (output power of excitation light). The upper limit value is a maximum output of the excitation laser. The lower limit value of the output power of the excitation light indicates a threshold value for stably supplying the excitation light. For example, when the excitation laser has a maximum output of 200 mW, it is necessary to use excitation laser at an output power equal to or larger than 40 mW.
Because of the upper limit value and the lower limit value of the output power of the excitation light from the excitation laser P4, a limitation is imposed on a variable gain range of the Raman amplifier P1. In other words, a Raman gain corresponding to the lower limit value of the output power of the excitation light from the excitation laser P4 is a minimum gain of the Raman amplifier P1, and a Raman gain corresponding to the upper limit value of the output power of the excitation light from the excitation laser P4 is a maximum gain of the Raman amplifier P1.
In order to widen the variable gain range of the Raman amplifier, the minimum gain of the Raman amplifier may be reduced. A method using an optical attenuator provided at an excitation light output (for example, Patent Document 1) may be employed as a method of reducing the minimum gain of the Raman amplifier. By using the method, in the case of the lower limit value of the output power of the excitation light, the excitation light power is reduced by the amount of attenuation of the optical attenuator, and hence the lower limit value of the excitation light power guided to the main signal can be reduced. When the lower limit value of the excitation light power reduces, the minimum gain of the Raman amplifier also becomes smaller. However, according to the method, not only in the case where the output power of the excitation light is the lower limit value but also in the case where the output power of the excitation light is the upper limit value, the same attenuation occurs. Therefore, the variable range of the excitation light power cannot be increased.    [Patent document 1] JP 11-168255 A
Therefore, such a Raman amplifier is desired in which an excitation light source stably operates in a case of a minimum gain, and a wide variable gain range can be obtained.